Human IMP dehydrogenase (IMPDH) catalyzes the NAD-dependent oxidation of IMP to XMP, making it a key component in the synthesis of guanine nucleotides. Two isoforms of the enzyme exit and these two forms share approximately 84% sequence identity. There is evidence that inhibitors of type II IMPDH are effective antiproliferative agents, and may be useful in controlling some forms on cancer. It has also been shown that inhibitors if IMPDH have immunosuppressive activity. Future efforts to understand and utilize the therapeutic benefits resulting from the inhibition of IMPDH activity would be greatly aided by obtaining the structure of this enzyme.